1. Field of the Invention
The present invention relates to the field of aeronautics and, more specifically, to the field of manufacturing turbine blades for turbomachines.
2. Description of the Related Art
Turbomachine blades generally consist of three parts: an upper part (or shroud), a middle part (or airfoil) and a lower part (or root), the upper and lower parts being separated from the airfoil by a platform. The airfoil is designed to be positioned in the gas flow in order to extract work through expansion of the gases in the flow, enabling it to drive the rotor of the turbomachine. An essential element in the production of a blade is ensuring the correct orientation of the airfoil relative to the root because, as the root is attached to a disk and is therefore immobile relative to the rotor of the turbomachine, this orientation determines the position of the airfoil in the flow of gases and, consequently, its aerodynamic efficiency.
The first stage in the manufacture of a blade is generally the creation of a casting which has the finished dimensions as regards the airfoil but is only a blank as regards the root and the shroud. The root and the shroud must then be machined to give them their definitive shape. Machining the root, in particular, is very important because it is this process which dictates the correct orientation of the airfoil relative to the flow of air. It is important to carry out this step of machining the root without increasing the uncertainty regarding the positioning of the airfoil relative to the root, in particular by avoiding excessive machining tolerances being added to the tolerances linked with the production of the airfoil.
Machining the root is traditionally carried out by positioning the blade in a known reference frame, connected with the machine tool, and embodied by six contact points which act as stops, against which the blade must press. A reference plane, oriented parallel to the direction of the leading edge of the blade and defining the orientation of the airfoil of the blade relative to a face of the root thereof the purpose of which is to come into contact with the slot of the disk on which the blade is to be mounted, is generally defined from these six points. The optimum aerodynamic efficiency of the airfoil is obtained if machining its root results in this reference plane of the blade corresponding to an ideal plane of orientation of the blade, also defined relative to the same face of the root.
Some of the points of the blade which are in contact with the stops are embodied by a reference point on the outer surface of the blade, which reference point is generated during forging or casting. The next step, which is essential for achieving correct positioning of the root relative to the airfoil, involves clamping the blade so that it does not move during machining of the root and so that the sides of the bulb or fir-tree shape of the root are correctly oriented. One of the difficulties associated with this operation is due to the three-dimensional shape of the airfoil which has no planar surface against which a perfectly braced pressure can be applied.
One known technique involves mechanically clamping the airfoil in a reference frame embodied by six points linked with the machine tool by pressing a gripping part against the airfoil. Once the airfoil has the correct orientation, i.e. once it has been rotated through what is referred to as a preselection angle which will impart to the airfoil the correct angle of attack in the flow of gases in the turbomachine, a face of the root which will act as a spatial reference for all subsequent steps is machined first of all. The following steps of machining the shape of the root and then the shape of the shroud are then carried out by positioning the machined face of the root against an appropriate reference frame which is defined on the machine tool.
First of all, this technique does not guarantee perfect stability during machining of the face of the root acting as the spatial reference because the pressure, which is generally provided at the center of the suction face of the airfoil, is limited by the fact that it acts in only one direction. The pressure which can be applied is also limited by the strength of the airfoil and by the strength of the skin which represents the thickness of this suction face. Without sufficient pressure, the airfoil can move during machining; excessive pressure, however, would result in the suction-face surface being marked, which could be accompanied by deformation of the profile of this suction face, which is relatively thin. This technique then accumulates uncertainties regarding the position of the elements to be machined, since first of all a reference frame linked to the airfoil is used when placing the latter on the machine tool, then the root is machined in a reference frame linked to the machine tool, and finally the shroud is machined in a reference frame linked to a face of the root.
Another frequently used technique involves embedding the airfoil, or at least a substantial portion thereof, in an encasing block made of a low-melting point material, such as an alloy of tin and bismuth. A face of the blade root is machined first of all. This face then serves as a spatial reference for positioning the blade in a six-point reference frame. The airfoil is then embedded in the encasing block of low-melting point material. Thus, the issue of precise positioning of the airfoil is becomes that of the positioning of the encasing block which is designed to have planar surfaces which will act as reference planes for the subsequent machining steps. After machining of the root and the shroud, the block is removed by melting and the blade regains its normal outer shape. However, this method still has the drawback of increasing the uncertainties in the positioning of the root by adding the tolerances resulting from placing the block around the airfoil to the tolerances of manufacturing the airfoil.